Perpendicular magnetic anisotropy (PMA) is widely used in devices requiring out-of-plane magnetization including Spin Torque Magnetic Random Access Memory (STT-MRAM) that has been described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996). In STT-MRAM, a magnetic layer with PMA can serve as a free layer, pinned layer, reference layer, or dipole compensation layer. PMA layers are found in various designs of PMA spin valves, magnetic tunnel junctions (MTJs), in PMA media in magnetic sensors and magnetic data storage, and within other spintronic devices.
Compared with conventional MRAM, STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a current perpendicular to plane (CPP) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic free layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic free layer if the current density is sufficiently high, and if the dimensions of the multilayer are small.
For STT-MRAM to be viable in the 90 nm technology node and beyond, the ultra-small MTJs (also referred to as nanomagnets) must exhibit a MR ratio that is much higher than in a conventional MRAM-MTJ which uses a NiFe free layer and AlOx as the tunnel barrier layer. The critical current density (Jc) must be lower than about 106 A/cm2 to be driven by a CMOS transistor that can typically deliver 100 μA per 100 nm gate width. Furthermore, a ferromagnetic layer with a long retention time is important for device application. To achieve this property that requires a high thermal stability, a free layer made of PMA material is preferred in order to provide a high energy barrier (Eb) and high coercivity. Strong PMA character is induced along an interface of a CoFeB layer or the like and a metal oxide such as MgO, for example. A ferromagnetic free layer must be thin enough so that induced PMA overcomes in-plane anisotropy. Intrinsic PMA is realized in laminated stacks including (Co/Pt)n, (Co/Pd)n, and (Co/Ni)n where n is the number of laminations but the MTJ may suffer from a lower magnetoresistive (MR) ratio than when CoFe or CoFeB is used for the free layer and/or reference layer.
When a memory element uses a free layer with a magnetic moment lying in the plane of the film, the current needed to change the magnetic orientation of a magnetic region is proportional to the net polarization of the current, the volume, magnetization, Gilbert damping constant, and anisotropy field of the magnetic region to be affected. The critical current (ic) required to perform such a change in magnetization is given in equation (1):
                              i          c                =                                            α              ⁢                                                          ⁢              e              ⁢                                                          ⁢              VM              ⁢                                                          ⁢              s                                      g              ⁢                                                          ⁢                              h                _                                              ⁡                      [                                          H                                  k                                      eff                    ,                                                                                  +                                                1                  2                                ⁢                                  H                                      k                                          eff                      ,                      ⊥                                                                                            ]                                              (        1        )            where is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, is the reduced Plank's constant, g is the gyromagnetic ratio, Hkeff,∥ is the in-plane anisotropy field, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, spin polarized current must be as small as possible.
The value Δ=kV/kBT is a measure of the thermal stability of the magnetic element. If the magnetization lies in-plane, the value can be expressed as shown in equation (2):
                    Δ        =                                            M              S                        ⁢                          VH                              k                                  eff                  ,                                                                                            2            ⁢                                                  ⁢                          k              B                        ⁢            T                                              (        2        )            where kB is the Boltzmann constant and T is the temperature.
Unfortunately, to attain thermal stability of the magnetic region, a large net magnetization is required which in most cases would increase the spin polarized current necessary to change the orientation of the magnetic region.
When the free layer has a magnetization direction perpendicular to the plane of the film, the critical current needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in equation (3):
                              i          c                =                              α            ⁢                                                  ⁢            e            ⁢                                                  ⁢            M            ⁢                                                  ⁢            s            ⁢                                                  ⁢                          VH                              k                                  eff                  ,                  ⊥                                                                          g            ⁢                                                  ⁢                          h              _                                                          (        3        )            
The parameters in equation (3) were previously explained with regard to equation (1).
Thermal stability is a function of the perpendicular anisotropy field as shown in equation (4):
                    Δ        =                                            M              S                        ⁢                          VH                              k                                  eff                  ,                  ⊥                                                                          2            ⁢                                                  ⁢                          k              B                        ⁢            T                                              (        4        )            
In both of the in-plane and out-of-plane configurations, the perpendicular anisotropy field of the magnetic element is expressed in equation (5) as:
                              H                      k                          eff              ,              ⊥                                      =                                            -              4                        ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                                                          ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              χ              ,              ⊥                                                          (        5        )            where Ms is the saturation magnetization, d is the thickness of the magnetic element, Hk,χ,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,s is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic element. In the absence of strong crystalline anisotropy, the perpendicular anisotropy field of a magnetic layer is dominated by the shape anisotropy field (−4πMs) on which little control is available. However, by enhancing the surface (interfacial) perpendicular anisotropy component, the perpendicular anisotropy (PMA) field is increased. Although MTJ structures with reference layer/tunnel barrier/free layer configuration such as CoFeB/MgO/CoFeB deliver a high MR ratio, there is still a need to enhance the PMA field component in a MTJ for higher thermal stability while maintaining a high MR ratio.